The synthesis of small nuclear RNAs is regulated during sea urchin embryogenesis, relative to other small RNAs in particular 5S rRNA. A cell free system based on isolated nuclei active in RNA synthesis, supplemented with cell protein extracts derived from different stages will be used to detect control factors. The genes coding for the small nuclear RNAs will be isolated by recombinant DNA techniques and the gene organization determined. This DNA will be used as a probe to determine the pathway of biosynthesis of these RNAs.